1. Field of the Invention
This invention relates to integrated circuit packaging, technology, and more particularly, to a method of bonding a BGA Hall Grid Array) package to a circuit board without causing collapse of the BGA package against the circuit board.
2. Description of Related Art
BGA (Ball Grid Array) is an advanced type of integrated circuit packaging technology which is characterized in the use of a substrate whose front side is mounted with a semiconductor chip and whose back side is mounted with a grid array of solder balls. During SMT (Surface Mount Technology) process, the BGA package can be mechanically bonded and electrically coupled to a printed circuit board (PCB) by means of these solder balls.
FIGS. 1A-1C are schematic sectional diagrams used to depict a conventional method for bonding a BGA package to a circuit board.
Referring first to FIG. 1A, this method is used to bond a BGA package 100 to a circuit board 110. The BGA package, 100 is formed with a plurality of bond pads 101 on the back side thereof (which are hereinafter referred to as package-side bond pads); while the circuit board 110 is formed with a plurality of corresponding bond pads 111 (which are hereinafter referred to as board-side bond pads), each being associated with one of the package-side bond pads 101.
During the SMT process, the first step is to prepare a grid array of solder balls 120, which are typically made of Pb/Sn 37/63 (which stands for 37% of lead and 63% of tin), and which are disposed between the package-side bond pads 101 and the corresponding board-side bond pads 111.
Referring further to FIG. 1B, in the next step, a solder-reflow process is performed by heating the solder balls 120 at the melting point of the solder balls 120, so as to melt the solder balls 120 to make them reflow over the board-side bond pads 111. Through this process, the solder balls 120 are wetted to both the package-side bond pads 101 and the board-side bond pads 111, thereby bonding the BGA package 100 to the circuit board 110.
One problem in the forgoing SMT process, however, is that, since the solder balls 120 are highly deformable when melted, the fixation of the BGA package 100 above the circuit board 110 should be carefully maintained; otherwise, the BGA package 100 would easily collapse against the circuit board 110.
As illustrated in FIG. 1C, in the event that the BGA package 100 is pressed down, or the BGA package 100 is unflatly shaped in its back side, it would undesirably cause the melted solder balls 120 to be crushed down and spread out beyond the board-side bond pads 111 which may even cause the solder balls 120 to come into contact with adjacent ones and thus become short-circuited to each other.
The U.S. Pat. No. 5,841,198 entitled xe2x80x9cBALL GRID ARRAY PACKAGE EMPLOYING SOLID CORE SOLDER BALLSxe2x80x9d teaches the use of solid-core solder balls to prevent the collapsing of BGA package against circuit board during the solder-reflow process. The utilization of this patented technology, however, has the following drawbacks. First, the solid-core solder balls are considerably more expensive to manufacture than conventional homogenous solder balls, so that it would make the overall packaging process much more costly to implement. Second, since these solid-core solder balls are hardly collapsible, it requires the BGA substrate surface to be highly planarized, which would make the overall packaging process more difficult to carry out.
The U.S. Pat. No. 5,591,941 entitled xe2x80x9cSOLDER BALL INTERCONNECTED ASSEMBLYxe2x80x9d teaches the use of high melting temperature solder balls together with low melting temperature solder paste to provide reliable bonding between BGA package and circuit board. This patented technology, however, is not intended to prevent the collapsing of BGA package against circuit board during solder-reflow process.
It is therefore an objective of this invention to provide a new method for bonding a BGA package to a circuit board, which can help prevent the collapsing of BGA package against circuit board during solder-reflow process.
It is another objective of this invention to provide a new method for bonding a BGA package to a circuit board, which can help prevent the solder balls from short-circuiting to adjacent ones during solder-reflow process.
In accordance with the foregoing and other objectives, the invention proposes a new method for bonding a BGA package to a circuit board.
Broadly recited, the method of the invention comprises the following steps: (1) preparing paring a grid array of solder tats, including: a fist group of solder balls of a specific reflow collapse degree; and a second group of solder balls of a specific reflow collapse degree less than the reflow collapse degree of the first group of solder balls; (2) arranging the first and second groups of solder balls in an interspersed manner in the grid array; (3) attaching the first and second groups of solder balls to corresponding bond pads on the BGA package and the circuit board; and (4) performing a solder-reflow process to reflow the first and second groups of solder balls over their associated bond pads on the circuit board.
In the first preferred embodiment, the first group of solder balls are homogenously made of a solder material of a specific melting point; and the second group of solder balls are each of the type including an outer portion and a core portion, with the outer portion having substantially the same melting point as the first group of solder balls, and the core portion being greater in melting point than the outer portion.
In the second preferred embodiment, the first group of solder balls are homogenously made of a first solder material of a specific melting point; and the second group of solder balls are homogeneously made of a second solder material whose melting point is greater than the melting point of the first group of solder balls.
During the solder-reflow process, when the first group of solder balls are entirely melted, the second group of solder balls are only partly melted (in the case of the first preferred embodiment) or entirely unmelted (in the case of the second preferred embodiment); and therefore, the second group of solder balls are capable of providing a firm support to the BGA package to prevent the collapsing of BGA package against circuit board during solder-reflow process.